An experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease is critical for the study of human AIDS. Simian immunodeficiency virus (SIV) induces an immunodeficiency syndrome in infected macaques that is remarkably similar in pathogenesis to human AIDS. An important use of this animal model system is the detailed study of pathogenesis and viral determinants of disease since many studies of this type are not feasible in humans. The purpose of this project is to investigate host and viral factors involved in variable disease progression in SIV-infected macaques and the lack of disease in African primates infected with their own strains of SIV. PATHOGENESIS OF SIVsm-INFECTION OF MACAQUES: To investigate the role of host factors in SIV-infection of macaques, we used a well-defined molecularly cloned virus (SIVsmE543-3). Our initial studies focused on defining the immune and viral factors involved in rapid disease progression. This small subset of SIV-infected macaques fails to maintain SIV-specific immune responses and progress to AIDS in less than six months from the time of inoculation. RP macaques showed profound and irreversible depletion of memory CD4+ T cells that likely explain the immune deficits in these animals. Molecular studies of virus in tissues and plasma of rapid progressor macaques exhibited common substitutions in the env gene. These substitutions were unusual in that they involved residues that were generally conserved and that were known to affect binding of env to CD4 or coreceptor. Virus from RP macaques was molecularly and biologically characterized and full length infectious molecular clones of various RP viruses were derived from plasma of a RP macaque, H635. The H635 viruses replicated less efficiently in primary macaque PBMC and macrophages than the parental SIVsmE543-3 strain and were much more senstitive to neutralizing antibody. Macaques inoculated with one of these viruses, H635-FC developed moderate to high primary and setpoint viremia, and progressive CD4+ T cell depletion but did not develop rapid disease. Sequence analysis of plasma virus in these animals demonstrated rapid loss of RP-specific env mutations concurrent with the development of neutralizing antibody response. These studies suggested that rapid progressor mutations are the consequence, not the cause of rapid disease progression. ROLE OF AUTOANTIBODIES IN AIDS PATHOGENESIS. The progressive decline of CD4+ T cells is a hallmark of disease progression in human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV)-infection. Whereas the acute phase of the infection is dominated by virus-mediated depletion of memory CD4+ T cells, chronic infection is often associated with a progressive decline of total CD4+ T cells, including the nave subset. The mechanism of this second phase of CD4+ T cell loss is unclear and may include immune activation induced cell death, immune-mediated destruction and regenerative or homeostatic failure. We studied patterns of CD4+ T cell subset depletion in blood and tissues in a group of 20 rhesus macaques inoculated with derivatives of the pathogenic SIVsmE543-3 or SIVmac239. Phenotypic analysis of CD4+ T cells demonstrated two patterns of CD4+ T cell depletion, primarily affecting either nave or memory CD4+ T cells. Progressive decline of total CD4+ T cells was observed only in macaques with nave CD4+ T cell depletion (ND), though the depletion of memory CD4+ T cells was profound in macaques with memory CD4+ T cell depletion (MD). ND macaques exhibited lower viral load and higher SIV-specific antibody responses and greater B cell activation than MD macaques. Depletion of nave CD4+ T cells was associated with plasma antibodies autoreactive with CD4+ T cells, increasing numbers of IgG-coated CD4+ T cells and increased incidence of autoreactive antibodies to platelets (GPIIIa), dsDNA and phospholipid (aPL). Consistent with a biological role of these antibodies, these latter antibodies were accompanied by clinical features associated with autoimmune disorders, thrombocytopenia, and catastrophic thrombotic events. More importantly for AIDS pathogenesis, the level of autoreactive antibodies significantly correlated with the extent of nave CD4+ T cell depletion. These results suggest an important role of autoreactive antibodies in the CD4+ T cell decline observed during progression to AIDS. SIV NEUROPATHOGENSIS SIV and HIV are both associated with the development of encephalitis. For HIV, the onset of AIDS dementia is generally a late stage finding. In contrast, most models of SIV encephalitis (SIVE) use animals that progress rapidly to disease. We observed a small number of conventional progressors that developed SIVE. The evolution of SIV in the cerebral spinal fluid (CSF) was compared with the virus that evolved in the plasma of two rhesus macaques that developed SIV encephalitis. While the virus in the CSF and plasma were similar during primary infection, distinct substitutions were observed sequentially in the two compartments. These findings are consistent with compartmentalization between the brain and blood during development of neuro-AIDS and the evolution of viruses with distinct genotypes and potentially distinct biological phenotypes in the brain. We are currently studying the pathogenesis of these CSF and brain isolates in a cohort of nave macaques and currently are observing development of SIV encephalitis in a subset of animals, as evaluated by increasing levels of SIV RNA in CSF samples. Evolution of virus in the brain, CSF, blood and lymphoid tissues will be evaluated. ASYMPTOMATIC INFECTION OF NATURAL HOST SPECIES A second goal of this project is to study the mechanisms underlying the lack of pathogenicity of SIV for their natural host species, with emphasis on SIVagm from vervet monkeys. SIVagm is capable of inducing AIDS in PT macaques but African green monkeys (AGM) do not develop overt signs of disease following infection. We evaluated the viral kinetics of a natural SIVagm isolate in two species of AGM, vervet monkeys (the species of origin of SIVagm90) and sabaeus monkeys from the Barbados. The virologic outcome in sabaeus and vervet AGM was surprisingly divergent. Inoculation of sabaeus AGM with SIVagm90 resulted in low and variable levels of primary viremia and low setpoints (<100 to 10,000 copies/ml). In contrast, inoculation of vervet AGM resulted in high primary viremia and establishment of moderate plateau levels (10,000 to 100,000). Regardless of the extent of viremia, CD4+ T lymphocytes remained stable throughout infection of AGM with no significant relationship between viral load and CD4+ T cell loss. A large number of investigations have shown that CD8+ T cell responses are critical for the containment of AIDS viruses in humans and Asian nonhuman primates. We compared the phenotype of T cell subsets and magnitude of SIV-specific CD8+ T cell responses in chronically SIVagm-infected vervet AGM and SIVmac-infected rhesus monkeys (RM). In comparison to RM, vervet AGM exhibited lower signs of immune activation and associated proliferation of CD8+ T cells as detected by granzyme B, Ki-67, and PD-1 staining. SIV Gag and Env-specific immune responses were detectable at variable but slightly lower levels in vervet AGM by IFN-g ELISPOT assay and intracellular cytokine staining than observed in RM. These observations demonstrate that natural hosts like SIV-infected vervet AGM develop SIV-specific T cell responses, but the disease-free course of infection does not appear to depend on the generation of robust CD8+ T cell responses.